LA County, contrary to what some might think, is not a desert.

Photo by Jules Miller on Unsplash

Its climate can be more accurately described as "Mediterranean" or "semi-arid." Which means that most of the time, it doesn't rain, except when it does, and then it rains a lot. That means the amount of water we have to go around varies dramatically from year to year - anywhere from 4 inches to over 40! This variability in precipitation presents challenges for reliably sustaining the 10 million or so people who reside within the county's 4,750 square miles. LA County gets roughly two-thirds of its water from a variety of outside sources, from the Sierra Mountains in Northern California to the Colorado River in the east. Delivering water to our taps requires a massive infrastructure network. Without it, LA County would still be a sparsely populated, dry place.

Where does your water come from?


Imported Water Sources

Northern California

Some 350 miles north of LA County, in between San Francisco and Sacramento, lies the California Delta (or the Sacramento-San Joaquin River Delta), a vast array of rivers and estuaries spread out over 1,100 square miles. Water from the Sierra Mountains flows down into the Delta and is carried south via the expansive State Water Project (or SWP), which begins at the Oroville Dam on the Feather River and ends at Lake Perris near Riverside. The SWP includes 22 dams and reservoirs in addition to the 444-mile California Aqueduct, and serves as a vital water source for counties across Southern California. It which provides water to irrigate over 750,000 acres of farmland in California, especially the San Joaquin Valley, and provides two-thirds of Californians some portion of their drinking water.

The Owens Valley

LA County's first and most controversial outside water source (see: LA Water Wars, or the movie Chinatown), the arid Owens Valley lies over 200 miles north of LA County, sandwiched in between the Sierra Nevada and White mountain ranges. Los Angeles acquired most of the water rights to the region at the start of the 20th century and subsequently built the 233-mile Los Angeles Aqueduct to carry the water. A 105-mile extension north to Mono Lake was constructed in the 1940s; another 137-mile section running parallel to the original aqueduct was added in the 1960s. Together, these aqueducts deliver an average of 430 million gallons of water every day, making up roughly a third of LA’s total supply.

The Colorado River

The 1,450-mile-long Colorado River’s expansive watershed includes portions of seven U.S. states, two Mexican states, several Indian Reservations and 11 National Parks. It provides stunning vistas, whitewater rapids, agricultural irrigation and water for metropolitan areas throughout the Southwest. A portion of LA County’s imported water comes from the river via the Colorado River Aqueduct, which carries the water 242 miles from the Parker Dam on Lake Havasu, at the California / Arizona border, to customers in LA County and other Southern California communities.

Local Water Sources

Roughly one third of our water supply here in LA County lies under the earth’s surface, bound up in layers of hard soil and clay in what are called groundwater basins, which serve effectively as underground reservoirs.

Groundwater Illustration


The ground beneath our feet is a mix of rocks, soil, air, and in some places, water. Water can seep through the ground and be stored in the spaces between soil particles. This water is called "groundwater", and the layer of fully saturated soil is called an "aquifer". Water stored in aquifers can move between soil particles, almost like very slow underground rivers. As water flows through the soil, it gets filtered and cleaned, and can serve as a source of local fresh water. Groundwater can be removed from an aquifer by pumping.

There are certain types of soils that don't allow water to flow through them easily - these soil layers are called impermeable layers. An aquifer with an impermeable layer above it is considered a "confined aquifer", meaning that it's more difficult for water to enter or leave the aquifer from above. An unconfined aquifer has no barrier above it, so water flowing along the surface of the ground can easily seep into the soil and become groundwater.

Groundwater is an important source of potable water for our region, but over-pumping can introduce problems with soil stability. When water is pumped out of the ground, it leaves behind pockets of air between soil particles. These pockets are air can cave in over time, causing the surface of the ground to subside.

Recharging, or replenishing, our groundwater is important for both our water supply and to protect against soil subsidence. In areas where the geology is conducive (i.e., surfaces above unconfined aquifers), water managers have created large pools of water that allow water to percolate down into the ground. These pools of water are called spreading grounds, and they play a vital role in making sure our local groundwater doesn't run out ? and that we have a reliable source of local freshwater for generations to come.

Engineering geologist measuring groundwater levels. (Kelly M. Grow / California Department of Water Resources)

Local Groundwater Basins

Groundwater reserves are in many places naturally refilled with rain that soaks into the ground, but can also be intentionally recharged using imported water, recycled water, and captured stormwater through a process referred to as enhanced replenishment. Not all areas of LA County have effective groundwater basins, but those that do sometimes rely on this source for up to 90% of their supplies.

Recycled water being used for lawn irrigation. (Kelly M. Grow, California Department of Water Resources)

Recycled Water

Much of the water that goes down the drain in our homes and businesses is piped away to water treatment facilities where it goes through several phases of cleaning, treatment, and testing before it is eventually released for reuse. Most recycled water in LA County is put to industrial uses; however, some of it is treated to an even higher standard of purification and used to replenish groundwater basins. The use of recycled water is promoting local sustainability and reducing our reliance on imported sources.

San Gabriel River Rubber Dam near Whittier Blvd. at the beginning of the San Gabriel Coastal Basin Spreading Grounds on Monday, April 8, 2013. (Diandra Jay / Los Angeles County Department of Public Works)

Stormwater Capture

Under normal circumstances, rainwater would fall to the ground and be absorbed by the earth, where much of it would make its way to underground aquifers, merging with the rest of our groundwater. Of course, in cities covered in concrete, it doesn't work out this way. Stormwater picks up pollution as it washes off surfaces, through our streets, into storm drains, and eventually spills out into local bodies of water and the ocean. An intricate and expanding system across LA County helps us capture or "harvest" stormwater so it can be treated and then used for purposes like replenishing our groundwater basins.


In the years to come, LA County’s water supply will face numerous challenges.

Leaks and water evaporation will continue to siphon off a significant amount of imported water as it travels through aqueducts, and the number of people living here continues to increase slowly and steadily. Most critically, climate change has resulted in intense heat and variation in our weather patterns. Water falls differently than it used to. Hotter hot days, more frequent and dryer dry spells, and wetter wet days are the new normal.

LA County’s water infrastructure is tasked with both delivering reliable water to our more than 10 million people and diverting stormwater to prevent flooding. As challenges to our water supply escalate, both our water management system and our own habits around water usage will have to adapt so we can maximize and conserve every last drop.